WASHINGTON (Detroit Free Press) — U.S. Rep. Justin Amash of west Michigan may have a national reputation as a libertarian lawmaker, but he's making some pretty powerful enemies back at home.

In the past week, both the Grand Rapids Chamber of Commerce and Right to Life of Michigan's political action committees — the latter representing one of the most powerful Republican-leaning groups in the state — have endorsed Amash's GOP opponent, businessman Brian Ellis.

Today, the Free Press learned first that the state Chamber of Commerce — representing businesses across Michigan — was endorsing Ellis as well, a rare rebuke of a sitting Republican congressman.

"Our decision primarily speaks to the strength of the challenger and our belief that he is a principled conservative who will be a pragmatic problem solver," said Richard Studley, president and CEO of the Michigan Chamber. "We believe Brian will be a more effective member of Congress than the incumbent."

Amash's campaign responded by saying that "Justin is 100% pro-life and the top conservative in Congress" and criticized Ellis' record as a member of a Grand Rapids-area school board and a state economic development board.

Amash, of Cascade Township, still has a lot in his favor in the race including, high name recognition, the status that goes with incumbency and the support of the DeVos family — widely considered Michigan's most influential Republican clan. He has more money in the bank and enjoys a sizable margin over Ellis in what little polling has been done in the 3rd district race.

But at a time when House Majority Leader Eric Cantor's primary defeat in Virginia on Tuesday is on everyone's mind in Congress, Amash may not be able to afford to ignore the forces amassing on Ellis' side, or their reasons for doing so. Primaries are often about getting the vote out and the Chambers and Right to Life are old hands at influencing — and often winning — campaigns.

"The (state) chamber brings its membership and money in many situations. Right to Life brings its list of activists," said Rich Robinson, with the Michigan Campaign Finance Network, a watchdog group. "Those are very important endorsements."

As the Free Press detailed recently, Amash may be riding high in libertarian circles but his votes — often against his own party's stated interests — have run afoul of many would-be supporters. Amash is meticulous about explaining his reasons for his votes on Facebook but at times those explanations, while earning praise from supporters who think government is run with too little attention to strict constitutional standards, fall short for some Republicans who believe his princples come at the expense of achievable policy goals.

Voting more against his party than all but one other Republican member of the House, Amash has compiled high marks from some conservative groups including Heritage Action, the American Conservative Union and the Club for Growth. But others have questioned his stands against GOP priorities such as Rep. Paul Ryan's fiscal 2012 budget plan and a balanced budget amendment.

It's not just those votes, however. A vote against defunding Planned Parenthood got him in trouble with anti-abortion groups like Right to Life of Michigan. Other votes, in which he was one of only a few in Congress voting against a breast cancer stamp or reauthorization of a poison control hotline, have earned a place in Ellis's "Amash Bizarre Votes" series of blog posts.

Members of the Meijer family, which owns stores across Michigan, and the political action committee for Dow Chemical are among the others actively supporting Ellis, a financial consultant who also served on a local Grand Rapids-area school board.

Ellis today said he was "honored" to receive the endorsements and again criticized Amash's vote against a Balanced Budget Amendment he considered flawed and a decision to remain neutral on legislation authorizing the Keystone pipeline, which Amash argued unconstitutionally helped one company.

"He is way out of touch with Michigan families and businesses," said Ellis, who Amash has said "isn't a serious or credible candidate," in recent remarks to WOOD-TV in Grand Rapids.

Studley said the Grand Rapids' chamber's decision played a big role in the state chamber's choice, as did concerns of business people in west Michigan.

"They've indicated to us he's been a great disappointment and he's not been an effective member of Congress," said Studley. "They want their members of Congress to be principled conservatives, but in west Michigan they don't just send people to Congress to vote no."

Bill Ballenger, contributing editor of Inside Michigan Politics, a newsletter in Lansing, said the chamber and Right to Life have lost before — such as when they endorsed then-state Attorney General Mike Cox for governor in 2010's primary — but that was a different, more crowded race.

Whomever wins the Republican primary in the 3rd will be heavily favored to win in this conservative districct.

"It (the endorsements) represents a serious threat to Justin Amash, but he must have seen this coming," said Ballenger. "I think he's prepared himself as well as possible."